Chip components joined to substrates are subject to crack formation along the bond lines between attachment structures (such as bumps) and the component, or between attachment structures and the substrate to which a chip component is attached. Cracks may occur in manufacturing or use when the materials comprising a component and its associated packaging materials expand and contract at different rates during thermal variation due to different coefficients of thermal expansion (CTE). Such cracks are a major source of device failure in chip components. Underfill techniques and materials are extensively used in semiconductor manufacturing to stabilize chip components and help prevent device failure.
While several types of underfill technologies are known, one of the most commonly used is ‘capillary underfill’. Capillary underfill typically comprises a flowable adhesive material dispensed adjacent to a substrate, proximate to an edge of a chip component so that the flowable adhesive contacts the component and the substrate, and is drawn into and through an intervening gap by a wicking action. When functioning properly, the underfill will migrate completely beneath the component, displacing all air and reaching to other edges of the chip component. The underfill may then be cured, whereby the underfill forms a substantially rigid material surrounding and strengthening each attachment joint, enabling the materials to better withstand the stresses applied to attachment structure bond lines during thermal variation. At each edge of the component, a generally concave fillet of underfill material may form, extending from the component to the substrate surface only a short distance beyond the peripheral boundary of the component.
After dispense, a flowable adhesive may flow not only into and through the gap between the component and the substrate, but also tends to flow away from the dispense point and the component, across the surface of the substrate. Once cured, this ‘counter-directional’ flow forms an ‘extended fillet’ of underfill material. This generally unimpeded flow away from the component necessitates, in many instances, the use of more underfill than is necessary to simply fill the component-substrate gap, and the underfill material comprising the extended fillet is essentially wasted, as it serves no substantially beneficial purpose. Further, the area covered by the extended fillet is generally designated a ‘keep out zone’ into which no other components are placed. Therefore, on the side of a chip component where an extended fillet of underfill material forms, the substrate surface area beneath the extended fillet is made unavailable for component placement, and is also essentially wasted.
Substrates, for example, printed circuit boards (PCB) used in computer motherboards, are typically designed and manufactured for extremely dense component placement, necessitated by the increasing performance demands of such devices, and also by the increasing market pressure to reduce the overall physical size of electronic devices. Therefore, consuming substrate surface area with an extended fillet of underfill, and therefore making that surface area off limits to component placement, complicates the already difficult device design challenges. Further complicating matters are the ever rising demands on device performance and tighter power regulation requirements, which sometimes result in placing certain power regulation components (for example, capacitors) in close proximity to processing components, including some chip components. Again, an extended fillet of underfill material may frustrate the design goal of placing components close to a chip component on the surface area otherwise covered by an extended fillet, thereby frustrating attempts to meet device power regulation objectives.